1. Field of the Invention
This invention relates to a two-cycle engine with means for controlling a port timing in accordance with the engine speed and more particularly to drive means for operating the port timing control means.
2. Related Art
It has been proposed to control or vary an exhaust port timing in accordance with the engine speed to improve power output over an entire effective range of the engine speed. Japanese patent application No. 60-3722 discloses one conventional port timing control device shown in FIG. 1 which comprises a pair of first and second port timing control valves 100 and 102 movable into and out of their respective closed positions where the first and second valves 100 and 102 cooperate with each other to close an upper portion of an exhaust port 104 to thereby delay an exhaust timing, that is to say, the opening of the exhaust port 104 by a piston 106 reciprocally moving along a cylinder 108, and drive means for moving the first and second valves 100,102 into and out of their respective closed positions along respective straight paths disposed perpendicular to the axis of the cylinder 108.
The drive means comprises an actuator 110 such as a motor, a rod 112 movable along an axis thereof and having a pair of first and second rack portions 112a and 112b on opposite ends portions, a first lever 114 fixedly mounted at one end on a rotatable output shaft 110a of the actuator 110, a first pinion 116 fixedly mounted on the upper end of the output shaft 110a of the actuator 110 and disposed in mesh with the first rack portion 112a, a second lever 118 fixedly mounted at one end on a rotatably shaft 120, and a second pinion 122 fixedly mounted on the upper end of the shaft 120 and disposed in mesh with the second rack portion 112b. Each of the first and second levers 114 and 118 has a slot 124 at the other end in which a pin 126, secured to each of the first and second valves 100 and 102, is slidably received. When the engine speed reaches a predetermined level, the actuator 110 is operated to rotate or angularly move its output shaft 110a about its axis, so that the first lever 114 is angularly moved counterclockwise as indicated by an arrow A (FIG. 1) to move the first valve 100 out of its closed position, that is, away from the exhaust port 104. At the same time, the second lever 118 is angularly moved clockwise as indicated by an arrow B (FIG. 1) through the first pinion 116, the rod 112 and the second pinion 122 to thereby move the second valve 102 out of its closed position in synchronism with the movement first valve 100, so that the exhaust port 104 is fully opened. The conventional drive means has been found not entirely satisfactory, however, in that the width L of the engine is increased since the rod 112 and the first and second levers 114 and 118 are considerably extended in the direction of the width of the engine, so that the overall size of the engine is increased. In addition, where the engine having such a relatively large width is mounted on a motorcycle M (FIG. 2), a cover member C, attached to the side of the cylinder block 128 at a position corresponding to the rod 112, is extended in the direction of the width of the motorcycle M to limit a space available for accommodating an exhaust pipe P. And, if the exhaust pipe P is arranged to pass over the cover member C as shown in FIG. 2, then a space for accommodating a radiator R is limited. As a result, the size of the radiator R and hence its capacity must be limited.
Further, since the drive means is composed of not a few component parts, there are occasions when the first and second valves 100 and 102 can not be brought accurately into their respective predetermined closed positions even if these component parts are manufactured within tolerance limits. Therefore, it has been desired to provide means by which the respective closed positions of the first and second valves 100 and 102 can easily be adjusted properly.